1. Field of the Invention
This invention relates to subgrade contouring devices and more particularly, to subgrading machines for precisely levelling and contouring the subgrade within an area having first and second parallel-oriented forms prior to placement of plastic concrete into the area.
2. Description of the Prior Art
The present invention represents a significant improvement of the subgrading machine disclosed in U.S. Pat. No. 4,249,327 (Allen). That patent discloses a subgrading attachment for a triangular truss concrete screed having a frame length greater than the spacing between the parallel-oriented concrete forms which support substantially the entire weight of the screed. A subgrading blade includes a length less than the spacing between the forms and is maintained in a position well below and in front of the screed by a multiple-element bracket. A winch system provides a traction force to the screed frame having an effective horizontal force vector aligned with the front screed blade at an elevation just above the upper surface of the concrete forms and well above the subgrade surface. As this subgrading machine is advanced into the uncontoured subgrade, a substantial drag force is imparted to the subgrade blade at an elevation substantially below the elevation of the forward traction force. The vertical spacing between the traction force vector and the drag force vector creates a lever arm and a resulting torque force which causes the entire subgrading machine to rotate forward as the device is translated into the unfinished subgrade. When using this prior art subgrading machine to contour densely compacted subgrade material, typically one or two workers stand on the rear screed blade to prevent undesired forward rotation or tipping of the machine as it is advanced into the unfinished subgrade material.
FIGS. 7, 11 and 17 of U.S. Pat. No. 4,316,715 (Allen) discloses an adjustable extension bracket for a vibratory concrete screed which can be coupled to the end sections of the screed at a selectable vertical position and which can be laterally telescoped with respect to the screed end section. This adjustable extension bracket is typically used where the forms or curbs on which the screed rides are positioned at a higher elevation than the elevation of the slab to be finished.